Being excellent in heat resistance, mechanical strength, or size precision, a curable resin represented by a phenol resin, an unsaturated polyester resin, or an epoxy resin has been used widely in various fields. On the other hand, a molded product obtained from a curable resin such as an epoxy resin has a problem that it shows a very fragile characteristic because of low fracture toughness.
To solve the above-mentioned problem, so far, techniques of dispersing polymer particles with a core-shell structure in a curable resin such as an epoxy resin have been disclosed (e.g., refer to WO2004/108825). Disclosed in these documents is a production method of an epoxy resin composition obtained by dispersing rubber-like polymer particles (B) in an epoxy resin (A), which is characterized in that an organic solvent (C) showing partial solubility in water is brought into contact with an aqueous latex of the rubber-like polymer particles (B) and thereafter an organic solvent (D) showing less partial solubility in water than the solvent (C) is further brought into contact with the particles (B) to substantially separate a water layer from the rubber-like polymer particles (B), a dispersion (F) consisting of the obtained rubber-like polymer particles (B) and the mixed organic solvents (C) and (D) are mixed with the epoxy resin (A), and volatile components are removed.
However, since the rubber-like polymer particles (B) are swollen in the epoxy resin composition in the above-mentioned method, the viscosity of the epoxy resin composition becomes high and it sometimes results in deterioration of the workability at the time of producing a molded product. Therefore, to suppress the viscosity of an epoxy resin composition low, the content of the rubber-like polymer particles (B) has to be suppressed.